The word Grengam in the name of the battle is a romanization of Гренгам, which is the traditional Russian cyrillization of Gränhamn, an older spelling of Granhamn. The spelling Gränhamn was used on Hans Hansson's map of Åland from about 1650.[11] In Sweden the battle is known as the Battle of Ledsund after the Ledsund strait.[8] In Finland the name Battle of Flisö (Finnish: Flisön meritaistelu) is used.[3]

The Swedish and Russian accounts of the battle differ significantly. Both sides agree that on July 27, 1720, a group of Swedish ships under Vice Admiral Carl Georg Siöblad attacked the Russian fleet and, in a pitched battle, had their four frigates captured by Russian sailors.

Contemporary Swedish drawing of the battle, showing the Swedish ship of the line and frigates closing in on the Russian galleys.

The Swedish squadron consisting of a 52-gun ship of the line, four frigates and nine smaller craft with a total of 156 guns and over 1,000 marines, made an attempt to attack the moving Russian fleet. General Mikhail Golitsyn managed to take an advantageous position in the narrow and shallow strait of Flisesund and ordered his ships into a semicircle formation. The Swedish ship of the line and four frigates entered the strait in pursuit of the Russian ships. Two frigates ran aground, making maneuvering for the rest of the squadron difficult.

In the fierce battle that followed, all four Swedish frigates were boarded. The only ship that managed to escape was Siöblad’s flagship.[5]

A small Swedish naval unit sailed right into the mighty Russian fleet anchored at Granhamn. A fierce battle took place, the Swedes lost their four frigates after they had run aground[12] but the Russian losses became so heavy that the entire fleet quickly decided to withdraw from Åland, leaving 43 sunken ships and 1000 dead Russians behind.[13] The Russian losses prevented their navy from launching any further major operations until the war ended with the Treaty of Nystad the following year.

Both sides claim the outcome of the battle as their own victory. They agree only in that four Swedish frigates, the 34-gun frigateStor Phoenix, the 30-gun Vainqueur, the 22-gun Kiskin and the 18-gun Danska Örn were captured by the Russians. No significant naval battles took place between the Russian and Swedish navies after this one until Sweden's defeat in the war was sealed by the Treaty of Nystad.

The Swedes lost 103 killed and 407 captured. The Russians had 82 killed and 236 wounded. The Battle of Grengam demonstrated the skillful use of the rowing fleet in the skerries environment and efficient reconnaissance and selection of the combat site, as well as perfect timing for the attack from different directions. The victory at Grengam allowed the Russians to consolidate in the archipelago, which was very important for carrying out operations against Swedish shipping in the area.[5]

Four Swedish frigates ran aground and were captured by the Russian Navy.[12] 43 out of 61 Russian galleys, were either sunk by the Swedish force or burnt and abandoned after the battle. The Swedish Vice Admiral Carl Georg Siöblad was initially criticised after the battle, but when the scope of the Russian losses were discovered he was praised. Russia celebrated the battle as a victory, but their fleet was unable to launch effective operations until the end of the war in 1721.

Like the previous Battle of Gangut, the Battle of Grengam was fought on Saint Pantaleon Day. In order to commemorate the perceived victory, a timber church to this saint was built in Saint Petersburg in 1722. It was rebuilt in stone in 1735-39. Since 1914 the facade of the church has borne two marble plaques listing the ships and regiments that fought at Gangut and Grengam.